fix: lock meta RMW and harden concurrent remix paths

Prevent worker/web clobbering of meta variants via flock and merge-by-id,
make filter timeouts thread-local, harden job-id/stem sanitization, migrate
TemplateResponse API, and remove the compare-bg experiment.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/main.py

@@ -30,7 +30,10 @@ _JOB_ID_RE = re.compile(r"^[A-Za-z0-9._-]+$")
 
 
 def _validate_job_id(job_id: str) -> str:
-    stem = pipeline.sanitize_stem(job_id)
+    raw = job_id or ""
+    if not raw.strip() or "/" in raw or "\\" in raw or ".." in raw:
+        raise HTTPException(status_code=400, detail="Ungueltige Job-ID")
+    stem = pipeline.sanitize_stem(raw)
     if not _JOB_ID_RE.match(stem):
         raise HTTPException(status_code=400, detail="Ungueltige Job-ID")
     return stem
@@ -82,8 +85,9 @@ def list_jobs() -> list[dict[str, Any]]:
 @app.get("/", response_class=HTMLResponse)
 def index(request: Request) -> HTMLResponse:
     return templates.TemplateResponse(
+        request,
         "index.html",
-        {"request": request, "jobs": list_jobs(), "site_title": config.SITE_TITLE},
+        {"jobs": list_jobs(), "site_title": config.SITE_TITLE},
     )
 
 
@@ -100,9 +104,9 @@ def job_detail(request: Request, job_id: str) -> HTMLResponse:
     )
 
     return templates.TemplateResponse(
+        request,
         "job.html",
         {
-            "request": request,
             "site_title": config.SITE_TITLE,
             "job_id": stem,
             "status": status,
@@ -166,9 +170,9 @@ def remix_form(
             }
 
     return templates.TemplateResponse(
+        request,
         "remix.html",
         {
-            "request": request,
             "site_title": config.SITE_TITLE,
             "job_id": stem,
             "options": options,